This particular safety plug was literally crumbling. The rubber was aged and eaten away from it's edges. I'd started a load of jars through the canner and air kept escaping around the plug. I tried (foolishly) to pack a towel over it and weight it down, but it just soaked the towel with steam. I shut the canner off and let it cool down, then finished my canning with my other canner.
Next time we were in town I stopped at the Hardware store and showed them my plug. They sold me a box with half a dozen plugs in it for a few dollars.
The years passed and then one of my canners started leaking around the seal. I tried holding the handles down tight, to stop the escape of steam. It worked sometimes. While I held it down tight the pressure would build and then the canner sealed and I was able to continue that load of jars.
I tried oiling the seals to buy time. These are the old seals made of rubber. Oiling did work...for a while. Then the seals were so stretched it was hard to keep them in the lid as I put it on the canner. sometimes it took a few tries to get the lid on with the seal in place. That was stupid. I should have gone straight to the hardware store and bought new seals.
Even though it seems like the canner is up to pressure and the weight eventually jiggles and everything seems to be going right, it's possible those old seals or plugs are not really letting the canner reach it's proper pressure, and without that, the contents of the jars may not reach a high enough temperature to safely preserve the food.
Nowdays I take that seriously. As preppers we should always have spare parts stored away for when we need them, but for anyone who uses pressure canners (or anything else important to their survival), it's nice to have the parts on hand when you need them! Then you don't have to stop in the middle of canning a batch of something to run to the store to buy what you need.
My canners are made my Mirro and have the model number stamped on the bottom. It's good that they do, because it's not in the book that came with my canners. I hunted all through the book and finally found a page with replacement part numbers.
Amazon, Pressure canner seals, safety plugs, and other parts
They're considerably cheaper than the hardware store, and more likely to have the part you need in stock.
Winter is a great time to look over your seals and safety plugs, and see if you need to order any replacements. It's a good idea to order extras to stock up and have on hand.
When my extras arrive I check to see that I have the right ones, then I put them back in their box and vacuum seal them in food saver bags to slow down oxidation. Then, just like the food that I preserve, I store them in cool, dark places. Heat and light can break down the material just like it does to food.
Some newer seals are made of silicone or plastic and don't need as much care to maintain them. The old rubber seals like my canners use can dry out and crack over time. The old wives tale of soaking them in water isn't really effective. Rubber doesn't absorb water. If you feel the need to "soften" or preserve them, spread a thin layer of cooking oil (any type) on them. Wipe excess oil off with a paper towel before use. The mirro company warned (in my instruction book from the 1980's) not to over-do the oiling of the rubber seals, but didn't explain why.
My standard practice for storing my canners over the winter was to put a light layer of oil on the seal and the safety plug.
I have a vacuum sealer now, but I don't use it for this because I never know when I'll randomly acquire something to can, or decide to do up a batch of beans (from dry) or something. So I still just put them in a ziplock bag.
Then I toss it inside the canner and put the lid on. My mother always said not to store the canner with the seal in place and the lid on tight. Again, she never said why. My speculation would be that it might compress the seal, sitting in storage, and reduce it's lifespan.
I store the pressure weights elsewhere, in a safe location in my kitchen. They're too expensive to replace. Around ten years ago I misplaced mine and it was going to cost $30 just for that little round weight that sits on the spindle on top of the canner. Now they've come down to around $20. (Canner weight for my Mirro Canner)
It's no badge of honor to be able to brag that you got ten years out of a gasket seal for your canner, or that you've never had to replace the safety plug. Canning is more than a hobby, or a necessary activity for food preservation or survival. It's a serious issue of safety and proper management. Be sure to include extra parts in your storage, and store them for the longest shelf life possible.